The invention relates to a method for operating an internal combustion engine, especially of a motor vehicle, wherein fuel is injected directly into the combustion chamber in a first operating mode during a compression phase and in a second operating mode during an induction phase and wherein the fuel injected into the combustion chamber is ignited. The combustion chamber is delimited by a piston. Furthermore, the invention relates to an internal combustion engine, especially for a motor vehicle. The engine includes an injection valve with which fuel can be injected directly into a combustion chamber delimited by a piston in a first operating mode during a compression phase and in a second operating mode during an induction phase. The engine includes a spark plug with which the fuel injected into the combustion chamber can be ignited and includes a control apparatus with which the injection valve and the spark plug can be controlled.
Systems of this kind for direct injection of fuel into the combustion chamber of an internal combustion engine are generally known. In this connection, a first operating mode, a so-called stratified operation, and a second operating mode, a so-called homogeneous operation, are distinguished. The stratified operation is especially used for smaller loads; whereas, the homogeneous operation is used for larger loads applied to the engine.
In stratified operation, the fuel is injected into the combustion chamber during the combustion phase of the engine and is injected there in the direct vicinity of the spark plug. This has the consequence that a uniform distribution of the fuel in the combustion chamber can no longer take place. The advantage of the stratified operation is that the smaller applied loads can be handled by the engine with a very low fuel mass. Larger loads can, however, not be satisfied by the stratified operation. In homogeneous operation, which is provided for such larger loads, fuel is injected during the induction phase of the engine so that a swirling and therefore a distribution of the fuel in the combustion chamber can easily take place. Thus, the homogeneous operation corresponds approximately to the operating mode of engines wherein fuel is injected into the intake manifold in the conventional manner.
In both operating modes, that is, in stratified operation and in homogeneous operation, the injection angle and the injection duration of the fuel to be injected are controlled (open loop and/or closed loop) to an optimal value by a control apparatus in dependence upon a plurality of parameters with respect to the following: fuel savings, toxic substance reduction and the like. This control (open loop and/or closed loop) ensures that, under normal operating conditions, the injection valve is closed when the injected fuel is ignited.
It is, however, possible that these normal conditions are no longer present, for example, because of a malfunction. Accordingly, it is possible that the high pressure pump is defective so that the fuel can be injected into the combustion chamber via the injection valve only by the rail pressure generated by the electric fuel pump.
In this case, the problem is present that the injection valve is still opened when the ignition of the fuel takes place. This would lead to damage of the injection valve.
It is an object of the invention to provide a method for operating an internal combustion engine with which a problem-free operation of the engine is possible.
This object or task is solved with the method of the kind initially mentioned herein or with an internal combustion engine of the kind initially mentioned herein in accordance with the invention in that the injection of the fuel is terminated in each operating state of the engine in advance of the ignition of the fuel.
In this way, it is ensured that the injection valve is closed in each case when the fuel is ignited by the spark plug. In this way, an early coking of the injection valve is reliably prevented. The operability and the service life of the injection valve is significantly increased thereby.
In an advantageous embodiment of the invention, the pressure is determined which is applied to the fuel to be injected. In this embodiment, the pressure present in the combustion chamber is determined and the injection of the fuel is ended in each operating state of the engine before or as soon as the pressure in the combustion chamber becomes greater than the pressure applied to the fuel.
The injection valve is closed at the latest at that time point in which the pressure in the combustion chamber becomes greater than the pressure applied to the fuel. In this way, it is achieved that no air/fuel mixture can be blown back from the combustion chamber into the injection valve. In this way, the operability of the fuel preparation system is maintained and a misfire or the like of the engine is avoided.
It is especially advantageous when the earlier one of the two cutoff angles is compared to the current rotational angle of the crankshaft and when an open injection valve is closed as soon as the current rotational angle of the crankshaft is equal to or greater than the selected earlier cutoff angle. In this way, it is ensured in each case that no injection valve is still open when the injected fuel is ignited in the combustion chamber or when the combustion chamber pressure is greater than the pressure operating on the fuel.
An advantageous configuration of the invention is characterized by the continuous repetition of the method of the invention at especially uniform time intervals or angular intervals. In this way, it is ensured that the method is carried out completely independently of all operating states of the engine so that, in each operating state of the engine, it is ensured that no damage to the injection valves takes place.
Of special significance is the realization of the method of the invention in the form of an electric control element which is provided for a control apparatus of an engine, especially of a motor vehicle. A program is stored on the electric control element which is capable of being run on a computer, especially on a microprocessor, and is suitable for executing the method according to the invention. In this case, the invention is realized by a program stored on the electric control element so that this control element, which is provided with the program, defines the invention in the same way as the method which the program can carry out.